Rocky Mountain spotted fever is the most frequently reported rickettsial disease in the United States. The etiologic agent, Rickettsia rickettsii, of this potentially fatal disease is transmitted to humans by the bite of infected ixodid ticks. Although, tick-transmitted spotted fever group (SFG) rickettsiae are maintained effectively in nature via transovarial transmission, very little is known about the molecular basis underlying rickettsial maintenance in ticks. The mechanisms by which rickettsiae propagate in tick tissues and their interaction with host tissues, including salivary glands, gut, and the ovaries are critical for our understanding of rickettsiae-tick interactions. Preliminary studies demonstrated that ticks and rickettsiae exist in a relationship in which the tick actively responds to the infecting rickettsiae. Although candidate molecules involved in this response have been identified, the extent to which rickettsiae interact with these tick-derived molecules is not known. Therefore, the goal of this proposal is to further determine the degree of interaction between SFG rickettsiae and ticks through molecular and functional characterization assays. Based on DNA sequence comparisons, candidate molecules putatively identified as membrane receptors (clathrin-coated VATPase, E-selectin precursor, GPV-CD42D) and cytoskeletal (VASP, alpha-catenin, and alpha-tubulin) components will be assessed for their utilization by rickettsiae. Recombinant proteins will be generated for selected candidate molecules and tested for interaction with rickettsiae by adhesion/invasion assays and GFP-reporter fusion protein utilization assays. These studies will further our understanding of both tick physiology and rickettsial transmission.